Cold-cathode discharge ion pump



Oct. 20, 1970 w. M. BRUBAKER ETAL 3,535,055

COLD-CATHODE'DISCHARGE ION PUMP Filed May 25, 1959 s sheets-sheen PRIORART PRIOR ART PRIOR ART INVENTORS.

Oct. 20, 1970 w. M. BRUBAKER ETAL COLD-CATHODE'DISCHARGE ION PUMP 3 Shets-Sheet a Filed May 25, 1959 v 5% a BM my m m5 5 5 5 V0 M v V m hwk \S.mQk .0 55 A E A .3 w o u 0 I m 3 W& Y B \3 $$$Q kk Filed May 25, 1959 3Sheets-Sheet 5 INVENTORJ J. Z m m J M m ,2 KM Mm m 0 7 Z .4 U m Y B 7 2United States Patent M US Cl. 417-48 13 Claims ABSTRACT OF THEDISCLOSURE An ion jump is described in which a cold cathode iondischarge is formed between an anode and a pair of opposed collectorswithin a pump casing. A source of vaporizable reactive material isprovided for depositing the reactive material in the pump casing.

This invention relates to ion pumps of the cold-cathode discharge type.More particularly, the invention relates to an improved cold-cathodedischarge ion pump. The invention is particularly useful for evacuatingnoble gases and other generally nonreactive ions.

Ion pumps may be generally classified into hot cathode types andcold-cathode discharge types. US. Pat. No. 2,850,225, issued Sept. 2,1958 to R. G. Herb, relates to a hot cathode ion pump. A pump of thetype referred to in U.S. Pat. No. 2,850,225 is sold by ConsolidatedVacuum Corporation, the assignee of the present application, under thetrade name Evapo-Ion. Such pumps are generally of high capacity, buthave the relative disadvantage of being somewhat bulky and expensive.

A cold-cathode discharge device for measuring the pressure in anevacuated space is described in US. Pat. No. 2,197,079, issued Apr. 16,1940, to Frans Michel Penning. The Penning device consists essentiallyof a ring anode contained between two cathodes within an evacuatedenvelope. The anode and cathodes are immersed in a strong magneticfield. A high positive potential with respect to the cathode is appliedto the anode. The gas between the anode and cathodes is ionized,allowing a current fiow therebetween. The magnitude of the ionizationcurent indicates the pressure existing between theanode and cathodes.Such a vacuum measuring device has the disadvantage of disturbing thevacuum to be measured, as ions are pumped from the evacuated space intothe surface of the cathode of the device, where they are collected. Thevacuum to be measured is thereby increased over the vacuum which wouldotherwise exist.

A practical vacuum pump can be made by combining a number of suchPenning cold-cathode discharge devices into a single unit. A descriptionof such a pump is contained in Science, vol. 128, pp. 282-284, publishedAug. 8, 1958. A pump comprised simply of a plurality of Penningcold-cathode discharge devices suffers from a poor noble gas evacuationcharacteristic, in that the evacuation pressure obtained by use of sucha pump when pumping noble gases oscillates instead of remainingconstant. The poor noble gas evacuation characteristic of such a-pumpresults from the inability of the pump surfaces to getter the ions ofnoble gases pumped thereinto. By getter is meant to establish physicalor chemical bonds between atoms and molecules of the pump surfaces andthe ions removed from the gas. The failure of the pump surfaces togetter the ions of noble gases is due to the nonreactive nature of suchions.

Nonreactive ions, when driven to the surfaces of the ion pump by thefields of force existing therein, are entrapped physically in theoutermost molecular layers of Patented Oct. 20, 1970 the pump surfaces.This entrapment consists of the ions being driven beneath the surfacemolecular layers of the pump so as to be physically retained by theenclosing molecular layers. No appreciable physical or chemical bondingexists between the entrapping molecular layers and nonreactive ions soentrapped. However, the entrapped ions are substantially neutralizedduring entrapment.

The neutralization of the entrapped ions changes the entrapped ions intoentrapped atoms of the nonreactive substances. If the entrappingmolecular layers are removed, these nonreactive atoms return to theevacuated area since no bonding exists. Return of the nonreactive atomsto the evacuated area causes a rise in the pressure existing within theevacuated area, because of the addition to the evacuated area of theatoms formerly entrapped.

Entrapped atoms are freed from entrapment by a process known assputtering. By sputtering is meant the ejection of particles of asurface in various directions from the surface. Sputtering is caused bythe bombardment of the surface being sputtered by ions traveling at ahigh velocity. As the pump surfaces are sputtered, the molecular layersof surface material entrapping the nonreactive atoms are worn away. Thenonreactive atoms thereupon escape back into the evacuated area.

The oscillatory nature of the nonreactive ion pumping characteristic ofconventional cold-cathode discharge ion pumps is apparently due to acumulative or avalanche effect in sputtering. When the surface molecularlayers have entrapped all of the nonreactive atoms which they arecapable of holding, further ion bombardment frees collected atoms fromthe portions of the surfaces they strike. The freeing of these atoms hastwo immediate effects. First, the pressure in the evacuated area risesin relation to the number of atoms freed. Second, the increase in thenumber of atoms in the evacuated area increases the sputtering rate ofthe collector surfaces, since more ions are available to strike thesesurfaces.

An increase in the sputtering rate of the ion collecting surfaces freesnonreactive atoms entrapped therein at a faster rate. The pressurewithin the evacuated area therefore containues to rise until the rate ofremoval of nonreactive atoms from the collector surfaces due tosputtering no longer exceeds rate at which the collector surfaces areable to entrap nonreactive ions. Thereupon, the collector surfaces againentrap the bombarding nonreactive ions until-the entrapment capacity isreached. Further nonreactive ion bombardment thereupon commences to freeentrapped nonreactive atoms at a rate faster than the rate at whichother nonreactive ions are entrapped, repeating the above describedprocess. Thus, the oscillatory nonreactive ion evacuation characteristicof the conventional cold-cathode discharge ion pump is not due to aninability to pump nonreactive ions into the collector surfaces of thedevice, but is rather due to an inability of the collector surfaces toretain such ions pumped thereinto.

According to the present invention, the oscillatory noble gas or othernonreactive ion evacuation characteristic of cold-cathode discharge ionpumps is eliminated by depositing the replacement material on thesputtered portions of the collector surfaces, into which the nonreactiveions are being pumped. This deposited replacement material providesadditional entrapping capacity for the sputtered collector surfaces.

A pump constructed according to the present invention has at least oneanode, at least one collector, and a repacement material sourcepositioned adjacent each collector, all within an evacuated envelope.Particles of replacement material from the replacement material sourceare caused to be deposited on the sputtered collector surfaces,preferably at a rate at least substantially equal to the rate ofover-all loss of material from the collector due to sputtering, therebymaintaining the entrapping capacity of the pump.

The invention may be more readily understood by reference to theaccompanying drawing in which:

FIG. 1 is an elevation of a Penning cold-cathode discharge device;

FIG. 2 is a sectional plan view, partially broken away, of a known ionpump consisting of a plurality of Penning cold-cathode dischargedevices;

FIG. 3 is an elevation, partialy in section, taken along line 33 of FIG.2;

FIG. 4 is a graph showing the oscillatory evacuation pressurecharacteristic of the ion pump of FIG. 2 when evacuating argon;

FIG. 5 is a sectional elevation of an improved coldcathode discharge ionpump according to one embodiment of the invention, in which material issputtered from sputter-cathodes and deposited on sputtered portions ofthe collectors;

FIG. 6 is a graph showing the improved evacuation pressurecharacteristic of the pump of FIG. 5;

FIG. 7 is a cross section of another embodiment of the inventionutilizable with magnetic fields which are uniform over a long distancein a direction parallel to the field, in which material is sputteredfrom sputter-cathodes and is deposited on sputtered portions of thecollector;

FIG. 8 is a sectional elevation of another embodiment of the inventionin which the collectors have transparent portions along the axis of thedischarge and the sputtercathode is positioned adjacent thesetransparent portions; and

FIG. 9 is a sectional elevation of another embodiment of the inventionin which the replacement material is evaporated from a replacementmaterial electrode positioned between the anode and an adjacentcollector.

Referring to FIG. 1, the Penning cold-cathode discharge device consistsof an evacuated envelope 20 containing an anode 21 and two collectorcathodes 22. The anode 21 is connected by a lead 23 to a high voltageterminal 24 extending through the evacuated envelope. The two collectorcathodes 22 are connected by a common lead 25 to a cathode terminal 26extending through the evacuated envelope. An electromagnet 27 creates amagnetic field between the anode 21 and the collector cathodes 22. Aninlet 28 connects the device to an evacuated space.

The anode 21 has a ring configuration so that electrons may pass throughthe anode with a relatively small chance of striking the anode surface.The collector cathodes 22 are of solid construction, so that ionsreaching the collector cathodes will strike their surfaces. A highpositive potential from a high voltage source (not shown) is applied tothe anode terminal 24, and the cathode terminal 26 is grounded through acommon connection (not shown). Electrons in the evacuated envelope 21will therefore tend to move toward the anode due to the attractionbetween the positive potential of the anode and the negative electroncharge.

As an electron tries to approach the anode 21, the magnetic field set upby the magnets 27 is such that the electrons spiral between the anodeand collector cathodes rather than continue to move directly toward theanode. During this spiralling process, the electrons produce ionizationin the area of the magnetic field by striking free molecules and atomsof the gas contained in the envelope. These ions are attracted to thecollector cathodes 22 and, upon striking one of the collector cathodes,impinge in the surface molecular layers of the collector cathode. Theremoval of these ions from the gas phase in the evacuated envelopetherefore reduces the pressure within the envelope.

FIG. 2 shows a conventional ion pump which consists essentially ofthirty-six Penning cold-cathode discharge cells. The ion pump of FIG. 2has a cellular-shaped anode 30 contained within an evacuated envelope31. An anode lead 32 connects the cellular-shaped anode 30 to ahighvoltage connector 33.

FIG. 3 shows an elevation, partially in section, of the ion pump of FIG.2. Two collector cathodes 34 are adjacent the upper and lower surfacesof the anode 30. A cathode terminal terminal 35 passing through theevacuated envelope 31 is connected to the two collector cathodes 34 by aconnecting lead (not shown). An inlet 36 is connected to the collectorspace to be evacuated.

The collector cathodes are constructed of a reactive material; forexample, titanium, magnesium, aluminum, molybdenum or various of therare earths may be used. A positive potential with respect to thecollector cathodes of approximately 3,000 volts is applied to the anode30 by means of the high voltage connector 33.

A gaseous discharge, occurring in the same manner as described abovewith respect to the Penning device, is initiated. The ions produced bythe gaseous discharge are driven into the collector cathode surfaces andentrapped under the surface molecular layers thereof. Some of thereactive ions are gettered by physical or chemical bonding with atomsand molecules of the collector cathode material.

Due to the high potential difference between the anode and the collectorcathode, ions strike the collector cathode with a relatively greatvelocity. Therefore, material on the surface of the collector cathodesis sputtered therefrom in the general direction of the anode and theopposite collector cathode. The removal of material from the collectorcathode surface exposes the entrapped nonreactive atoms in the nextsucceeding molecular layer. These entrapped atoms are thereby permittedto escape.

A portion of the material sputtered from one collector cathode isdeposited on the opposite collector cathode. Sputtered material is alsodeposited on the anode and on the envelope walls. Consequently, the rateof removal of material from a collector cathode due to sputteringexceeds the rate of deposit on the collector cathode of materialsputtered from the opposite collector cathode, resulting in an over-allloss of collector cathode material. Thus, it is apparent that after ashort period of operation, the oscillatory pumping characteristic occurswhen nonreactive ions, for example, of a noble gas, are being pumped.

FIG. 4 is a graph of the actual pressure measured in in a pump of thetype illustrated in FIGS. 2 and 3 when pumping argon, a noble gas. Apositive potential of 3,000 volts with respect to the collector cathodeis applied to the anode. The pressure within the evacuated envelopevaries from a minimum of 0.75 X 10 mm. Hg to a maximum of 2.5 x10 mm.Hg. The periods between maximum pressure peaks are approximately sixminutes.

FIG. 5 shows an improved cold-cathode discharge ion pump acording to theinvention. An evacuated envelope 50 contains a pair of collectorelectrodes 51 and a cellular anode 52. A collector electrode terminal 53passes through the evacuated envelope 50 and is connected to the twocollector electrodes 51 by a connecting lead (not shown). Neither thecollector electrodes nor the anode need be constructed of reactivematerial. An anode lead 54 connects the anode 52 to a high voltageconnector 55. A pair of cellular replacement material elements 56 whichfunction as sputter-cathodes are positioned between the anode and thecollectors. As shown in FIG. 5, there are nine sputter-cathode collectorcells for each anode cell, preferably aligned as shown so that thecenter of the anode cell is in alignment with the center of asputtercathode cell. A sputter-cathode lead 57 connects thesputter-cathodes 56 to a bias terminal 58 passing through the evacuatedenvelope 50. An inlet 59 connects the device to the space to beevacuated.

The sputter-cathodes are constructed of any material Which will sputtersatisfactorily. It is not essential that the sputter-cathodes 56 beconstructed of reactive ma terial, such as titanium. However, whenevacuating gases which may be gettered, it is preferable to use suchreactive materials, so as to increase the pump capacity by addition of agettering effect. The sputter-cathodes 56 are arranged so that thecellular passages extending therethrough are substantially perpendicularto the surfaces of the collectors 51. The cellular passages through thesputter-cathodes need not be aligned with the anode cell walls, althoughsuch alignment is preferable.

A positive potential with respect to the collector of from 2,000 to4,000 volts is applied to the anode 52 by means of the high voltageconnector 55. A negative potential of between 2,000 and 4,000 volts isapplied to the sputter-cathode 56 by means of the bias terminal 58. Amagnetic field of from 1,000 to 2,000 gauss is applied to the pump by amagnet 27.

Due to the cellular arrangement of the anode and sputter-cathodes, ionsmoving from the anode toward the collectors tend to assume a pathsubstantially parallel to the sides of the sputter-cathode passages, andrelatively few ions strike the sputter-cathodes. Due to the largepotential difference between the anode and the sputter cathodes, ionsapproaching the sputter-cathodes are accelerated, and ions striking thesputter-cathode do so with a relatively high velocity. These collisionscause the material of the sputter-cathodes to be sputtered in thedirections of the collectors.

Since-the ions strike the sputter-cathodes with a glancing or gougingmotion, a relatively greater amount of sputtering of material from thesputter-cathodes occurs per collision than occurs when an ion strikesthe collector surfaces in a substantially perpendicular direction.Therefore, a smaller number of ions striking the sputter-cathodessputter sufficient material therefrom to replace the over-all loss ofmaterial from the collector surfaces due to the sputtering caused by agreater number of ion collisions.

The following details are typical of the construction of an ion pumpaccording to the embodiment of the invention illustrated in FIG. 5. Theion pump has an anode and two collectors constructed of stainless steel.The sputter-cathode may also be constructed of stainless steel. If airor other gases which may be partially gettered are to be pumped, it ispreferable to construct the sputtercathode and collector of a reactivematerial so as to increase the pump capacity by gettering reactive ions,rather than depending on entrapping alone. The over-all thickness of thepump is 1% inches. The anode and each sputter-cathode are one inchsquare. The anode has 4 square cells. The sputter-cathode has 36 squarecells. The anode is 1 /6 inch thick and each sputter-cathode is inchthick. The space between a collector and the adjacent sputter-cathode isA inch. The space between each sput ter-cathode and the anode is inch. Apotential positive, with respect to the collector, of 3,000 volts isapplied to the anode and a potential negative, with respect to thecollector, of 2,000 volts is applied to the sputter-cathodes.

FIG. 6 is a graph of the evacuation pressure obtained with an ion pumpconstructed with the above dimensions. The substance being pumped isargon. It is to be noted that a substantially constant pressure ofapproximately mm. Hg exists in the evacuated envelope. The oscillatorycharacteristic illustrated in FIG. 4 has been eliminated. The minimumpressure shown in FIG. 4 is slightly below the average pressure shown inFIG. 6, indicating that the leak rate of the apparatus whose evacuationcharacteristic is shown in FIG. 6 was higher than the leak rate of thedevice whose evacuation characteristic is shown in FIG. 4.

FIG. 7 illustrates another embodiment of the invention. The embodimentof FIG. 7 is particularly useful in devices having magnetic fields whichare uniform over a long distance in the direction parallel to themagnetic field. A cylindrical collector 70 is annularly contained withina cylindrical anode 71, an annular 72 being formed thereby.sputter-cathode replacement material elements 73 extend outward radiallyfrom adjacent the collector 70 toward the anode 71.

A high positive potential with respect to the collector is applied tothe anode 71. A high negative potential with respect to the collector isapplied to the sputter-cathodes 73. Some of the ions moving toward thecollector 70 will sputter material from the sputter-cathodes 73 by aprocess similar to the process described with respect to the device ofFIG. 5. The replacement material sputtered from the sputter-cathodes 73is deposited on the surface of the collector 70, so as to providecontinuous entrapment of nonreactive ions pumped thereinto. Asatisfactory nonreactive ion pumping characteristic is thereby achieved.

The alternate embodiment of the invention illustrated in FIG. 8 isespecially useful when the magnetic field is uniform but short. It hasbeen found that the density of the ions causing sputtering is normallygreatest along the axis of the gaseous discharge. That is, the dischargeconforms generally to the outline of each anode cell, and sputtering isgreatest opposite the center of the anode cell. In the embodiment ofFIG. 8, those portions of the collectors which are aligned with the axisof the discharge are made transparent. The sputter-cathodes arepositioned on the axis of the discharge. Some of the ions, instead ofstriking the collectors, strike the sputter-cathodes and sputtermaterial therefrom. This sputtered material is deposited on thecollectors to provide the entrapping layers.

The device of PIG. 8 has an evacuated chamber containing a pair ofcollectors 81 and a cellular anode 82. A collector terminal passesthrough the evacuated envelope 80 and is connected to the two collectors81 by connecting leads 84. The anode 82 is connected by an anode lead 85to a high voltage connector 86. Each of the collectors 81 has a seriesof openings 87 aligned with the axis of the discharges of the anodecell. sputter-cathodes 88 are positioned so as to be adjacent theopenings 87. The positioning of the sputter-cathodes 88 causes materialsputtered therefrom to be deposited on the collectors. Thesputtercathodes 88 are connected by connecting leads to a bias terminal89 passing through the evacuated envelope 80. An inlet 59' connects theevacuated envelope 80 to the space to be evacuated.

A potential, positive with respect to the collectors 8 1, is applied tothe anode 82 through the high voltage terminal 86. A potential, negativewith respect to the collectors 81, is applied to the sputter-cathodes 88through the bias terminal 89. Ions passing through the collectoropenings 87 strike the sputter-cathodes 88, causing sputtering. Thematerial sputtered from one sputter-cathode is deposited primarily onthe opposite collector and sputter-cathode, due to the almostperpendicular angle at which the sputtering ions strike thesputter-cathode surface. Since the sputter-cathodes 8 8 are sputtered toa greater extent than are the collectors 81, due to their placementalong the discharge axis, material from the sputter-cathodes isdeposited on the collectors at a higher rate than the rate of over-allloss of material from the collectors due to sputtering. Thus, acontinuous deposition of an entrapping layer occurs on the collectors,thereby providing a satisfactory non-reactive ion pumping characteristicfor the cold-cathode discharge ion pump.

FIG. 9 illustrates an alternate embodiment of the invention in Whichmaterial is evaporated from replacement material electrodes and theevaporated material is deposited on the sputtered portions of thecollectors which in this embodiment functions as collector cathodes. Anevacuated chamber 90 contains a pair of collectors 91 and a cellularanode '92. The anode is connected by means of an anode lead 94 to a highvoltage connector 95. Two replacement material electrodes 96 arepositioned one adjacent each of the collectors 91 by a replacementmaterial electrode 96. The replacement material electrodes have leads 97passing through the collectors 91. The leads 97 are insulated from thecollectors 91 and the envelope 90. A battery 99 completes an'electricalcircuit for each replacement material electrode 96 so as to causeheating of the electrode 96.

Heating of the electrodes 96 evaporates material therefrom. Theevaporated material is deposited on the adjacent collector surfaces.These adjacent collector surfaces are the surfaces which are beingsputtered by ion bombardment. The rate of evaporation of material fromthe electrodes 96 is controlled by the potential of the batteries 99 sothat replacement material preferably is deposited on the sputteredcollector surfaces at a rate substantially equal to the rate of over-allloss of material from the collectors 91 due to sputters. Thus, acontinuous film of collector replacement material is deposited on thecollectors 91 by the evaporation of replacement material from thereplacement material electrodes 96. The replacement material electrodes,anode, and collector, may be const-ructed of any suitable material. Ifreactive ions are being pumped, use of reactive materials for thecollectors and replacement material electrodes is preferable, so as toadd a gettering effect.

We claim:

'1. An improved ion pump of the cold-cathode discharge type comprisingan anode, a collector positioned adjacent the anode, a source ofvaporizable replacement material positioned between the anode and thecollector, means for heating said source to evaporate replacementmaterial therefrom, means for initiating a voltage gradient within atleast a portion of the space between the anode and collector and theextending in a direction from the anode toward the collector, means forenclosing said anode, said source and said collector so as to provide asealed enclosure during. pump operation, and means for producingmagnetic lines. of force through the enclosure wherein the lines extendin a direction from said anode toward said collector.

2. An ion pump as defined in claim 1 wherein said replacement materialcomprises a reactive material.

3. An ion pump as defined in claim 1 wherein the anode has a cellularconfiguration.

4. An improved ion pump of the cold-cathode discharge type comprising acellular anode and a collector, enclosure means containing said anodeand collector, means for initiating a gaseous discharge in the spacebetween the anode and the collector, a replacement material elementpositioned between the anode and the collector so that materialevaporated from the replacement material element during pump operationis deposited on portions of the collector, means connected to thereplacement material element to cause material to be evaporatedtherefrom at a rate at least substantially equal to the rate of over-allloss of material from the collector due to cathode sputtering, and meansfor producing magnetic lines of force through the enclosure wherein thelines extend in a direction from said anode toward said collector.

5. An ion pump as defined in claim 4 wherein said replacement materialcomprises a reactive material.

6. An improved ion pump comprising an anode, a collector adjacent theanode, a source of vaporizable replacement material positioned adjacentthe collector, means for heating said source to evaporate replacementmaterial therefrom, means for initiating a voltage gradient within atleast a portion of the space between the anode and the collector andextending in a direction from the anode toward the collector, anenvelope for enclosing said anode, said source and said collector andbeing in communication with a space to be evacuated, and means forproducing magnetic lines of force extending from said anode toward saidcollector.

7. An ion pump as defined in claim 6 wherein said replacement materialcomprises a reactive material.

8. An ion pump as defined in claim 6 wherein the anode has a cellularconfiguration.

9. An electronic vacuum pump comprising an anode, a cathode, a vacuumpump casing enclosed with said anode and cathode, means for causing amultiple coldcathode discharge between said anode and cathode in saidcasing, means independent of said last named means for depositingreactive material within said casing, and in inlet to the interior ofthe casing.

'10. An electronic vacuum pump comprising anode means defining aplurality of separated discrete glow discharge regions, a cathode, avacuum pump casing enclosing said anode means and cathode, means forcausing a multiple cold-cathode discharge between the anode means andcathode in said casing, means independent of said node means and saidcathode for depositing reactive material within said casing, and anoperating communicating with the interior of the casing.

11. In an electronic vacuum pump, anode mean defining a plurality ofseparated glow discharge regions, means for producing and directing amagnetic field along a predetermined axis in said regions, cathode meansdisposed transversely of said axis, a vacuum-tight envelope containingsaid anode and cathode means, means for causing a cold cathode dischargein each of said regions between said anode and cathode means, meansseparate from said discharge means for supplying and depositing reactivematerial within said envelope, and an inlet to the interior of saidenvelope for communication with a chamber to be evacuated.

12. In an electronic vacuum pump for operation in a magnetic fieldestablished in a predetermined direction, in combination, cathode meansdisposed in said magnetic field and transversely to said direction,anode means disposed in cooperative relation with respect to saidcathode means and constructed to define a plurality of separated glowdischarge regions between said cathode and anode means, a vacuum tightenvelope containing said anode and cathode means, means for causing acold cathode discharge in each of said regions between said anode andcathode means, means separate from said discharge means for supplyingand depositing reactive material within said envelope, and an inlet tothe interior of said envelope for communication with a chamber to beevacuated.

13. An ion vacuum pump comprising a pump envelope adapted to beconnected to an enclosure to be evacuated, an anode mounted within saidenvelope, a collector surface mounted adjacent said anode, means forproducing magnetic lines of force extending in a direction from saidanode toward said collector, means including said anode for producing agaseous discharge in the magnetic field within at least a portion of thespace between the anode and the collector for ionizing gases therein byaccelerating electrons (within said portion and thereby bombard gasmolecules therein, said last means being adapted to produce a potentialgradient within said portion of said region for directing at least someof the ions toward said collector surface, a source of vavorizablemetallic substance positioned within said envelope, and means forvaporizing said metallic substance some of which is adapted to becondensed upon portions of said collector surface.

References Cited UNITED STATES PATENTS 2,636,664 4/1953 Hertzler 230-69X 2,727,167 12/ 1955 Alpert. 2,796,555 6/ 1957 Connors 23069 X FOREIGNPATENTS 797,232 6/1958 Great Britain.

WILLIAM L. FRE'BH, Primary Examiner UNITED STATES PATENT OFFICECERTIFICATE OF CORRECTION Patent No. 3,535,055 Dated October 20. 1970Inventor) Wilson M. Brubaker et al It is certified that error appears inthe above-identified patent and that said Letters Patent are herebycorrected as shown below:

Column 1 line 13, "jump" should read pump line 32 "Evapo-Ion" shouldread Evapor-Ion Column 5, line 75, "annular" should read annulus Column6, line 72, after "91" insert so that the anode 92 is separated fromeach collector 91 Column 7, line 13, "sputters" should read sputteringline 30, before "collector" insert the Column 8 line 16 "node" shouldread anode line-l7 "operating" should read opening same column 8, after"References Cited" insert 2 ,894 ,679 Herb; 2 ,850;225 Herb;

2,888,189 Herb; 2,913,167 Herb; 3,080,104 Vanderslice; 2 ,993,638 Hallet al 2 ,197 ,079 Penning Signed and sealed this 20th day of April 1971(SEAL) Attest:

EDWARD M.FLETCHER,JR. WILLIAM E. SCHUYLER, JR. Attesting OfficerCommissioner of Patents FORM USCOMM-DC 60376-P69 lLS Govt-"MINT 'IlnTlMGOFFICE: I", O 3'33.

